In a 16-month feeding study male and female CD-1 mice received semi-synthetic diets containing 0, 1.5 or 3.0 ppm T-2 toxin. Feed consumption, body-weight gains, clinical findings (including haematological examinations at 16 months) and the development of external lesions were recorded. At 3, 6, 12 and 16 months, animals were killed for assessment of their immune function. Disease-related deaths did not differ among groups. Histological examination of all organs revealed statistically significant differences from controls in the incidence of pulmonary adenomas and hepatic adenomas in the 3.0-ppm group. Other treatment-related findings were an increased prevalence of epithelial hyperplasia in the forestomach of animals treated with T-2 toxin, and increased heart weights in treated male mice. T-lymphocyte-dependent humoral immunity tests did not reveal treatment effects and haematology revealed no particular trends. It is concluded that chronic feeding of T-2 toxin at low levels is not immunosuppressive but has a carcinogenic or tumour-promoting effect in mice.